How can I see the actual numerical output RGB values of the cycles 'Blackbody' node? (preferably through api)
Example for the compositor node which is present by default:

import bpy

scene = bpy.context.scene
scene.use_nodes = True

nt = scene.node_tree

#get the node
node = nt.nodes['Composite']

#access by string key in 2.69
input = node.inputs['Alpha'].default_value = 0.5

#older versions of blender might require
#access by index
for index, input in enumerate(node.inputs):
    if input.name == "Alpha":
        print(index)
        break

#link    
output = nt.nodes['Render Layers'].outputs['Alpha']
nt.links.new(input, output)


You can hover over the input with python tooltips enabled:


Using python it is only possible to setup the tree and its parameters.

Via OSL you can implement your own shaders. Enable Open Shading Language in the render menu, add a script node and see Text Editor -> Templates -> Open Shading Language

Template Temperature To RGB


shader temperature_to_rgb(
    float Kelvin = 1200.0,
    output color Color = 0.8)
{   
    /* Kelvin to RGB */
    Color = blackbody(Kelvin);
}


As you can see blackbody is an inbuilt function of the standard library.